1. Field of the Invention
This invention relates to a radiation image read-out and reproducing system for obtaining an image signal by reading out a radiation image from a recording medium, such as a stimulable phosphor sheet, on which the radiation image of an object has been recorded, carrying out image processing of the image signal, and reproducing a visible image by using the processed image signal. This invention particularly relates to a method of adjusting a radiation image read-out condition to be used for image read out and/or a radiation image processing condition to be used for image processing of the image signal.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value designed so as to match the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and by using the image signal the radiation image of the object is reproduced as a visible image on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed, by setting an appropriate read-out gain when detecting the emitted light and converting it into an electric signal to be used in reproducing a visible image on a recording material or a display device.
In order to detect an image signal with an appropriate read-out condition in accordance with a radiation dose to a stimulable phosphor sheet and the like, it has been proposed to constitute a radiation image recording and reproducing system such that preliminary read out is carried out for approximately ascertaining the radiation image stored on the stimulable phosphor sheet by scanning the stimulable phosphor sheet with a light beam of a comparatively low level of energy, and a preliminary read-out image signal obtained by the preliminary read out is analyzed. Thereafter, final read out is carried out for obtaining the image signal, which is to be used for reproducing a visible image, by scanning the stimulable phosphor sheet with a light beam having a higher level of energy than the level of energy of the light beam used in the preliminary read out, and reading out the radiation image with a read-out condition adjusted to an appropriate value on the basis of results of analysis of the preliminary read-out image signal.
The term "read-out condition" as used herein means generically various conditions affecting the relationship between the amount of light emitted by the stimulable phosphor sheet during image read out and the output of a read-out means. For example, the term "read-out condition" means a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or the power of stimulating rays used for image read out.
The term "level of energy of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of light emitted by the stimulable phosphor sheet depends on the wavelength of the light beam, i.e. has a distribution of sensitivity to the wavelength, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the level of energy of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity to the wavelength. In order to change the level of the light beam, light beams of different wavelengths may be used, the intensity of a light beam produced by a laser beam source or the like may be changed, or the intensity of a light beam may be changed by moving a ND filter or the like into and out of the optical path of the light beam. Alternatively, the density of scanning may be changed by changing the diameter of a light beam, or the speed of scanning with a light beam may be changed.
Regardless of whether the preliminary read out is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary read-out image signal) obtained and to adjust an image processing condition appropriately for use in image processing of the image signal on the basis of results of analysis of the image signal. The proposed method is applicable to the cases where an image signal is obtained from a radiation image recorded on a recording medium such as a conventional X-ray film, as well as to the system using the stimulable phosphor sheet.
One of the conditions which should be considered when adjusting a read-out condition and/or an image processing condition is to remove signal components which represent unnecessary image portions recorded on a recording medium, for example, an image portion upon which only scattered radiation impinged, and an image portion upon which radiation impinged directly without being passed through or reflected by an object. In this manner, it is necessary to adjust a read-out condition and/or an image processing condition so that only the portion which is to be viewed is reproduced with an appropriate image density when a visible image is ultimately reproduced on a photographic material or the like.
As an example of the methods described above, a method of adjusting a read-out condition disclosed in U.S. Pat. No. 4,527,060 will be described hereinbelow with reference to FIG. 1A. FIG. 1A is a graph showing a histogram of a preliminary read-out image signal SP. The disclosed method is used in a system wherein a stimulable phosphor sheet is utilized and preliminary read-out is carried out.
With reference to FIG. 1A, values of the preliminary read-out image signal SP, which is obtained by detecting light emitted by a stimulable phosphor sheet during preliminary read-out and which is proportional to the amount of the emitted light, are plotted on the horizontal axis (logarithmic axis). The frequency of occurrence of the values of the preliminary read-out image signal SP is plotted on the vertical axis at the upper part of the graph. Also, values of an image signal SQ obtained during final read-out are plotted on the vertical axis at the lower part of the graph (logarithmic axis). In this case, the histogram of the preliminary read-out image signal SP is composed of projecting portions A, B and C, and it is assumed that the projecting portion B corresponds to the portion necessary as a visible image reproduced ultimately. By way of example, in order to find out the projecting portion B, the histogram is searched by using a predetermined threshold value T from the position of the minimum value SL of the preliminary read-out image signal SP along the direction of increase of the image signal values, i.e. along the chained line. In this manner, a second rising point "a" and the next falling point "b" are found. The range sandwiched between the points "a" and "b" is found as the image information range which is to be reproduced into a visible image. The minimum value and the maximum value of the preliminary read-out image signal SP in the found image information range are denoted by Smin (which corresponds to the point "a") and Smax (which corresponds to the point "b") respectively. The disclosed method of adjusting a read-out condition adjusts the read-out condition for the final read-out so that Smin and Smax are detected respectively as the minimum image signal value Qmin and the maximum image signal value Qmax in the final read-out. The minimum image signal value Qmin and the maximum image signal value Qmax in turn correspond respectively to the minimum density Dmin and the maximum density Dmax within a correct image density range in the visible image ultimately reproduced on a photographic material (hereinafter referred to as a recording sheet). (Said minimum density Dmin and said maximum density Dmax will hereinbelow be referred to as the minimum density Dmin and the maximum density Dmax on a recording sheet.) More specifically, the read-out condition for the final read-out is adjusted so that the image information falling within the range from Smin to Smax is detected as values lying on the straight line G1 shown in FIG. 1A.
When the read-out condition for the final read-out is adjusted in the manner described above, the necessary image information alone can be detected with best resolution.
Also, in a system wherein no preliminary read out is carried out, the same thing as described above is carried out in the course of image processing. Specifically, it is considered that an image signal represented by the same histogram as the preliminary read-out image signal SP shown in FIG. 1A is obtained in an image read-out step which corresponds to the final read out in the system wherein preliminary read out is carried out. In this case, the image signal ranging from SL and SH shown in FIG. 1A is detected in the image read-out step. Thereafter, image processing is carried out so that the range which is sandwiched between Smin and Smax shown in FIG. 1A and which is to be reproduced into a visible image corresponds to the range sandwiched between the minimum density Dmin and the maximum density Dmax on a recording sheet.
As described above, a read-out condition and/or an image processing condition is adjusted so that the range sandwiched between the minimum value (i.e. Smin in preliminary read out, or Qmin in final read out shown in FIG. 1A) and the maximum value (i.e. Smax in preliminary read out, or Qmax in final read out shown in FIG. 1A) of the image signal necessary for reproducing a visible image corresponds to the whole range sandwiched between the minimum density Dmin and the maximum density Dmax on a recording sheet. As a result, a visible image with best density resolution can be reproduced by taking full advantage of the performance of the recording sheet.
However, in cases where the object is a periphery of a limb of the human body or is an infant by way of example, the range of the image signal necessary for reproducing a visible image is very narrow. (That is, the difference between the maximum density and the minimum density at a necessary portion of the object is small when the radiation image of the object is reproduced into a visible image.) When the visible image is reproduced so that the performance of a recording sheet is utilized to the fullest, a visible image having very high contrast and unsuitable for viewing purposes is obtained.
Furthermore, X-ray images recorded on X-ray films have approximately constant contrast regardless of the type of the object. Therefore, an observer who is experienced in viewing X-ray images having approximately constant contrast cannot readily adapt himself to an image having high contrast.